Sensory mechanisms of the vestibular system involve several different physiological response types of primary afferent neurons, based on adequate stimulus modality, response variability, adaptation and sensitivity. Peripheral structural diversity also is present in vestibular accessory structures, hair cell (receptor) ciliary bundles, hair cell internal ultrastructure, synaptic contacts and axon diameters. The objective of the proposed research is to determine relationships between some of these basic physiological properties and structural features. Otolith organs of fishes will be studied because, first, fishes have no cochlea or specialized acoustic papilla, and use their otolith organs for detecting tilt, vibration and sound; and second, because on fishes the three otolith organs have relatively uniform gross structure, and contain hair cells of only the ultrastructural Type II, thus eliminating many structural variables. Methods involve determining the function of identified afferents innervating hair cells in specific regions of each macula. Single-unit responses to tilt, vibration and sound will be analyzed from the goldfish, which has known regional diversity of the types of ciliary bundles in the sensory maculae. The fibers recorded from will be followed grossly in their branches to the areas they innervate, and intracellular dye injection will be tried to allow tracing axons to their terminals on the receptor cells, to see what structural correlates of function are present. Another approach to the same goal will be a scanning electron microscope study of the otolith organ maculae in elasmobranch fishes, in which regional localization of response types is known. Integrative relationships of fiber diameter, sensitivity and firing patterns will be studied in the goldfish, and electron microscopic observations will be used to determine properties of spatial convergence. Finally, some comparisons between species may suggest the evolutionary diversification of vestibular receptors.